A semiconductor wafer or substrate can be made with a variety of base substrate materials, such as silicon (Si), germanium, aluminum nitride (AlN), gallium arsenide (GaAs), gallium nitride (GaN), aluminum gallium nitride over gallium nitride (AlGaN/GaN), indium phosphide, silicon carbide (SiC), or other bulk semiconductor material for structural support. In particular, GaN is a binary III/V direct bandgap semiconductor material with properties of hardness (12±2 GPa), mechanically stable wide bandgap (3.4 eV), and high heat capacity and thermal conductivity. GaN and AlGaN/GaN can be doped with Si, or with oxygen to n-type and with magnesium (Mg) to p-type. The wide band gap allows the performance of GaN devices to be maintained up to higher temperatures (400° C.) than Si devices (150° C.) with lesser effects of thermal generation of charge carriers that are inherent to semiconductors. The high breakdown voltage, high electron mobility, and saturation velocity of GaN and AlGaN/GaN are suitable for high voltage, high power, high frequency, high temperature, and radiation applications. GaN and AlGaN/GaN is also widely used in optoelectronics and other applications requiring low resistance and low energy consumption.
A GaN or AlGaN/GaN semiconductor wafer or substrate (collectively referred to as GaN substrate) is commonly grown by atomic layer deposition to build up a GaN lattice structure on a thick Si wafer. Special care must be exercised during the manufacturing process to avoid cracking, breakage, or other structural damage to the GaN semiconductor substrate. To reduce stress and cost of the GaN lattice structure, it is typically made ultra-thin, on the order of 10-50 micrometers (μm).
The supporting Si tends to reduce the breakdown voltage of the GaN device, which is counter to the desired properties described above. The GaN substrate is often made thicker to compensate, which adds cost and stress factors. However, a higher breakdown for the GaN substrate would negate the need for a thicker GaN substrate to decrease cost and stress factors. To meet the breakdown voltage target, the Si wafer and Si layer must be removed before the final GaN device is realized. Due to the difficulty and expense of removing the silicon wafer, in most cases the silicon wafer is retained and the device performance suffers. In cases where the Si support is removed, the step is performed at the wafer-level, before with singulation of the GaN substrate. However, after silicon substrate removal, the ultra-thin GaN die are susceptible to cracking, breakage, or other structural damage during the subsequent device integration and packaging process.
In another micro transfer printed process, the GaN substrate is again grown by atomic layer deposition to build up a GaN lattice structure on a thick first Si wafer. A stamped wafer includes a plurality of protrusions on a bottom surface of the stamped wafer, matching one-for-one with the GaN die on the GaN substrate. The GaN die are undercut by an etching process to singulate the GaN substrate and then lifted off by contacting the protrusions of the stamped wafer to separate from the first Si wafer. The GaN die no longer have the support of the first Si wafer. The individual GaN die, as supported now by the stamped wafer, are transferred to and placed active surface oriented up on a second Si wafer. An interconnect structure is formed and routed from the second Si wafer up and over the active surface of the GaN die. Again, the first Si support is removed at the wafer-level, before or simultaneous with singulation of the GaN substrate. The protrusions of the stamped wafer provides limited stability and support and may not properly pick-up all GaN die. In addition, forming the interconnect structure from the second Si wafer up and over the active surface of the GaN die can lead to cracking, breakage, or other structural damage.